Monday, April 04, 2005

"I checked my engine, it was still there"

I get this question occasionally. "My check engine light came on, what do I do?"

A quick guide:

The "Check Engine" or ("MIL") light indicates that your engine computer has detected a fault in one of the engine controls or sensors, especially one of the controls related to emissions control. Examples include intake air temperature sensors, knock sensors, and oxygen sensors.

If your MIL is flashing, STOP DRIVING IMMEDIATELY (IF IT IS SAFE TO DO SO). Your engine is misfiring at a high rate. You can damage your catalyst and even your engine if you drive in this condition for more than a short time. Pull over and have the car towed to the nearest mechanic or to your dealer.

If your MIL is lit, but not flashing, drive normally. You won't harm your vehicle if it is running smoothly. When you have time, pull your battery cable, or the fuse which feeds your engine computer. Wait 30s, and then re-connect it. This will reset your computer. If you have a real problem, the MIL will come back, usually in several days.

Most problems that cause the MIL to light are not dangerous to drive with, however, you may be polluting the air a little more than the EPA or CARB would like. You also may find you have reduced fuel efficiency, rough idling, or reduced power.

Probably the most common cause for a MIL is a loose gas cap, which triggers an evaporative emissions system code. If you get a MIL, try tightening down the gas cap, and disconnect the battery as mentioned above.

If you are so inclined, you can get an OBD code scanner for less than $100, and read your diagnostic codes yourself. Then, you may be able ot pinpoint and repair your problem yourself. There is a list of OBD-II codes here.

4 comments:

Keep in mind that disconnecting the battery is often ineffective on newer cars, as DTCs are stored in nonvolatile EEPROM. Doing so might just result in setting a history code for low battery voltage :)

According to the latest regulations:If the fault has gone away, the lamp will turn itself off. For most faults it takes 3 fault free driving cycles for the lamp to turn off i.e. 3 cold start and drives. The fault will be erased from the fault memory after 40 or 80 driving cycles. As the previous poster said, removing the battery cable is unlikely to make any difference on a modern car.

If the lamp is on, it is on for a reason, it is better to find out why, rather than just blindly trying to turn it off. For most people this means a trip to the dealer.